The term “wireline formation tester” is the generic name in the petroleum industry for a wireline logging tool used for determining formation fluid pressure and other parameters in a reservoir. A prior art wireline formation tester typically includes a formation pressure tester tool having a probe with a pretest chamber and a hydraulically-driven pretest piston. A pressure sensor is coupled to measure tool pressure.
Measurement of formation fluid pressure by a formation tester may be repeated once or twice without changing the position of the probe. Proper placement of the formation tester requires lowering the formation tester into the well and pressing the probe of the pressure tester tool against the borehole wall. The measurement procedure includes a “draw-down” procedure followed by a “build-up” procedure.
Before drawdown, the probe is pressed against the mud cake on the borehole wall. During drawdown, a small amount of formation fluid (typically 10 cc) is extracted from the reservoir. The prior art draw-down procedure includes establishing hydraulic communication between tool fluid and formation fluid (by retracting the pretest piston in the pretest chamber to reduce the tool pressure and break the mud cake seal), verifying good hydraulic communication between tool fluid and formation fluid using the pressure sensor, and verifying good hydraulic isolation between tool fluid and borehole fluid using the pressure sensor.
Immediately following drawdown, the pretest piston is stationary in the retracted position and fluid in the pretest chamber is at a pressure below the pressure of formation fluid.
Build-up includes allowing a build-up period to establish pressure equilibrium between tool fluid and formation fluid. During build-up, the pretest piston remains stationary in the retracted position. Formation fluid flows from the formation into the tool because formation fluid pressure is higher than tool pressure. Continued inflow allows tool pressure to build up until equilibrium is established. When equilibrium is established, tool pressure equals reservoir pressure. The changing pressure in the tool is monitored by the pressure sensor. The build-up procedure includes waiting for equilibrium to be established; and setting pressure of formation fluid equal to the measured tool pressure.
When using wireline formation testers for determining formation fluid pressure, especially in low permeability formations, it is most desirable that equilibrium be established within a short time. If the formation tester is set at a particular location for too long a time, it could stick in the borehole and become difficult to remove. Fear of the tool sticking in the borehole is a major concern and is frequently cited as the main reason for not using wireline formation testers more often. For this reason, the tester is usually allowed to remain on the borehole wall for no more than a limited period of time. The limited period of time varies widely depending on the nature of the formation and the downhole borehole pressure, temperature, etc. Because wireline formation testers often fail to reach equilibrium within the time allowed, several data processing extrapolation techniques have been developed for estimating reservoir pressure from a time-series of pressure measurements. These techniques, to the extent they provide accurate estimates, avoid the need to wait for equilibrium to be established. However, these techniques are not generally viewed as reliable predictors of actual formation fluid pressure.